In many wireless communication systems, a frame structure is used for data transmission between a transmitter and a receiver. For example, the IEEE 802.11 standard uses frame aggregation in a Media Access Control (MAC) layer and a physical (PHY) layer. In a typical wireless station such as a transmitter, a MAC layer inputs a MAC Service Data Unit (MSDU) from upper layers and attaches a MAC header thereto, in order to construct a MAC Protocol Data Unit (MPDU). The MAC header includes information such as a source address (SA) and a destination address (DA). The MPDU is a part of a PHY Service Data Unit (PSDU) and is transferred to a PHY layer in the transmitter to attach a PHY header thereto to construct a PHY Protocol Data Unit (PPDU) for transmission to another wireless station such as a receiver. The PHY header includes parameters for determining a transmission scheme including a coding/modulation scheme.
In IEEE 802.11n WLAN communications, if legacy stations (L-stations) and high-throughput stations (HT-stations) coexist in the same WLAN, then frames of the HT-stations use a mixed mode PHY layer header which includes both a legacy PHY header part and a high-throughput (HT) PHY header part. FIG. 1 shows a legacy format frame 10 in which a legacy PHY header part 12 includes legacy training fields (L-TFs) 14 and a legacy signal field (L-SIG) 16. Further, an HT PHY header part 18 includes an HT signal field (HT-SIG) 20 and HT training fields (HT-TFs) 22. The frame 10 further includes a MPDU 24 containing data. A legacy duration or period 26 indicates communication of the HT-SIG 20, the HT-TFs 22, the MPDU 24 and a block acknowledgement field (BA) 28, over a wireless channel.
Legacy format frames can be successfully received by both the L-stations and the HT-stations. However, the L-stations cannot receive HT frames successfully because the L-stations cannot understand the HT PHY header part of the HT frames.
The frame format in FIG. 1 implements an extended PHY protection approach for an exchange of frames (packets) when the L-stations and the HT-stations coexist in the same WLAN. FIG. 2 shows an example of channel access in an IEEE 802.11n network based on a L-SIG Transmission Opportunity Protection (L-SIG TXOP Protection) approach using the frame format of FIG. 1. The L-SIG TXOP Protection approach uses the L-SIG 16 in the PHY header 12 to prevent legacy transmission of the contents of more than one HT format PPDU. A station (STA) such as a receiver STA may use L-SIG TXOP Protection for an exchange of packets by utilizing the L-SIG portion of an HT PPDU. With L-SIG TXOP Protection, a Network Allocation Vector (NAV) is used within IEEE 802.11n networks to prevent the STAs from accessing a shared wireless channel and causing contention. The NAV is maintained by each STA and is an indicator of time periods when transmission will not be initiated even though a Clear Channel Assessment (CCA) function of the STAs does not indicate traffic on the channel. A NAV duration value is virtually carried in the length and rate fields of the L-SIG.
L-SIG TXOP Protection provides robust protection for third party HT stations along with L-stations using HT PPDUs, enabling protection packets to be sent in optimized multiple-input-multiple-output (MIMO) PPDUs.
When L-SIG TXOP Protection is in effect, the length and rate fields of the L-SIG are set so that the end point of the legacy duration or period (i.e., the ratio of legacy length and legacy rate), is equivalent to the intended NAV duration by subtracting an Extended Interframe Space (EIFS) period 30 from a DCF Interframe Space (DIFS) period 32.
In IEEE 802.11n networks, when the channel (e.g., a radio link) has been free of any traffic for a period greater than the DIFS period 32, then the STAs may have immediate access to the channel in a contention-based service. The EIFS period 30 is longer than the DIFS period 32, wherein the EIFS 30 period is only used by a STA when there has been an error in frame transmission whereby the STA waits for an EIFS period 30 before trying to access the channel again. A L-station that decodes the L-SIG length and rate fields will continue receiving communications for the legacy duration, thereby preventing the L-station from starting communication over the channel during this EIFS period. This leads to unfairness for L-stations in gaining access to the channel relative to HT-stations.
To avoid unfairness towards L-stations, each L-SIG TXOP Protection duration (period) 34 is set according to an interval 36 that represents the difference between corresponding EIFS and DIFS periods (i.e., an EIFS-DIFS interval adjustment). The EIFS-DIFS interval 36 is shorter than the actual NAV protection period. This is intended to avoid unfairness towards the L-stations which defer for an EIFS period upon receiving a PPDU using L-SIG TXOP Protection (causing a Cyclic Redundancy Code (CRC) error). The HT-stations add the EIFS-DIFS interval to the L-SIG TXOP Protection duration when setting the NAV value for a PPDU that uses L-SIG TXOP Protection.
However, the EIFS-DIFS interval cannot solve the unfairness problem for the L-stations. This is because the EIFS period begins following an indication by the PHY layer that the channel is idle after detection of an erroneous frame, without regard to the virtual carrier-sense mechanism. Two conditions must be satisfied to start the EIFS period: (1) detection of an erroneous frame from a PHY or a MAC CRC verification, and (2) after detection, but before start of the EIFS period, the channel must be idle as indicated by a PHY CCA. After a L-SIG TXOP Protection sequence, HT-stations can contend for the channel earlier than L-stations. Therefore, the L-stations have lower probability of gaining access to the shared channel than the HT-stations. This causes channel access unfairness for L-stations.
FIG. 3 shows another example of channel access based on extended PHY protection (EPP). Each station may use EPP to provide protection for an exchange of packets, wherein the L-SIG in the PHY header is used to protect against legacy transmission of contents of more than one HT format PPDU.
In the example in FIG. 3, the last frame in the EPP sequence is transmitted as a legacy format frame. Since as noted above the L-stations cannot correctly receive that last frame, the duration in the L-SIG is set to the end of the EPP sequence in EPP duration 35, such that the L-stations will start an EIFS procedure at the end of the EPP sequence (e.g., L-SIG TXOP Protection sequence). However, this is still unfair for the L-stations since the HT-stations can start a DIFS procedure at the end of a L-SIG TXOP Protection sequence in a L-SIG TXOP Protection duration but the L-stations cannot.